After publishing, discontinue `genui_a2ui` and refer to `genui_a2a`

_↴ Ported from [flutter/genui#775](https://github.com/flutter/genui/issues/775) — originally opened by [gspencergoog](https://github.com/gspencergoog) on 2026-02-26._
_Original labels: P1_

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We've renamed the `genui_a2ui` package to `genui_a2a`, so we need to (on pub.dev) discontinue `genui_a2ui` and set the recommended replacement to `genui_a2a`, but not until `genui_a2a` has been published.

This is a pub.dev package management task. Locate the pubspec.yaml or similar configuration for the `genui_a2ui` package. The action is to mark it as discontinued on pub.dev and set `genui_a2a` as the recommended replacement. Verify that `genui_a2a` has been published first. The work is administrative and involves updating package metadata.

索引モデルが issue の本文から書いたものです。